Fix order of operations during rb_ec_finalize.

Merged: https://github.com/ruby/ruby/pull/3605

eval.c

@@ -162,7 +162,11 @@ static void
 rb_ec_scheduler_finalize(rb_execution_context_t *ec)
 {
     rb_thread_t *thread = rb_ec_thread_ptr(ec);
-    rb_thread_scheduler_set(thread->self, Qnil);
+    EC_PUSH_TAG(ec);
+    if (EC_EXEC_TAG() == TAG_NONE) {
+        rb_thread_scheduler_set(thread->self, Qnil);
+    }
+    EC_POP_TAG();
 }
 
 static void
@@ -217,6 +221,10 @@ rb_ec_cleanup(rb_execution_context_t *ec, volatile int ex)
 
     rb_threadptr_interrupt(th);
     rb_threadptr_check_signal(th);
+
+    // If the user code defined a scheduler for the top level thread, run it:
+    rb_ec_scheduler_finalize(ec);
+
     EC_PUSH_TAG(ec);
     if ((state = EC_EXEC_TAG()) == TAG_NONE) {
         th = th0;
@@ -278,9 +286,6 @@ rb_ec_cleanup(rb_execution_context_t *ec, volatile int ex)
 	}
     }
 
-    // If the user code defined a scheduler for the top level thread, run it:
-    rb_ec_scheduler_finalize(ec);
-
     mjit_finish(true); // We still need ISeqs here.
 
     rb_ec_finalize(ec);

test/fiber/test_scheduler.rb

@@ -35,4 +35,18 @@ class TestFiberScheduler < Test::Unit::TestCase
 
     thread.join
   end
+
+  def test_close_at_exit
+    assert_in_out_err %W[-I#{__dir__} -], <<-RUBY, ['Running Fiber'], [], success: true
+    require 'scheduler'
+
+    scheduler = Scheduler.new
+    Thread.current.scheduler = scheduler
+
+    Fiber.schedule do
+      sleep(0)
+      puts "Running Fiber"
+    end
+    RUBY
+  end
 end